Lithographic step-and scan projection systems are well known in the art. FIG. 1 is a schematic diagram of a conventional lithographic step-and-scan projection system 10. The step-and-scan system 10 comprises an illumination source 12 for generating an illumination beam, a mirror and lens system 16 for manipulating the illumination beam, an optical slit 24 for confining the illumination beam, and an imaging lens 26 for focusing the confined, illumination beam on a resist-coated wafer 20 to be printed. The step-and-scan system 10 may also comprise a tilting wafer stage 18 for holding and tilting the wafer 20 to be printed, and/or a tilting mask holder 22 for holding and tilting a mask having a mask pattern.
The lithographic step-and-scan projection system 10 of FIG. 1 is inherently suitable for superimposing images in a range of defocus from a given mask pattern, resulting in improvement of DOF. When images are superpositioned, the higher contrast in the focal image can make up for the lower contrast in the defocused images. Hence, when the image contrast at the focal plane is more than sufficient, there is room to raise the contrast of the combined image, to the contrast required for line width control. Continuous, defocal image superimposing is simply achieved by tilting either the mask 22 (FIG. 1) or the wafer 20 (FIG. 2) with respect to the optical path O during scanning.
However, improving the DOF with this continuous superposition of defocal images is less effective than simply superimposing two discrete defocal images. More specifically, FIG. 3 shows the exposure-defocus plot of a continuously superimposed contact-hole wafer image having a DOF on the order of 0.236 μm at 9.5% exposure latitude. The exposure conditions were set to numerical aperture (NA)=0.2, σ (sigma)=0.5, at a wavelength of exposure radiation (λ)=13.4 nm for contact hole at pitch=70 nm, and nominal critical dimension (CD)=35 nm. The parameter σ is an aperture ratio, i.e., the ratio of the NA of the condenser lens over the NA of the imaging lens, that indicates the degree of partial coherence of the illumination and imaging system. The larger the sigma, the lower the degree of coherence. As can be seen from FIG. 3, scanning with a tilted mask or wafer produced a range of continuous defocal wafer images of about 0.96 μm. On the other hand, FIG. 4 shows the exposure-defocus plot of a contact-hole image exposed using discrete superimposition of two, discrete contact-hole wafer images, which are at two different defocal positions. As can be seen, scanning using discrete superimposition of two images at a defocal distance of 0.48 μm between them, i.e., the distance between the two defocal positions, produced a DOF on the order of 0.415 μm, at substantially the same exposure energy latitude.
Accordingly, a method is needed for implementing discrete superpositioning of two or more defocal wafer images at different defocal positions in a lithographic step and scan projection system.